Bacterial chemotaxis, perhaps the most primitive behavioral system in biology, is nonetheless quite complex. Much has been learned about the structure of the motor organelle, the flagellum, in terms of its structural genes and its components. The sequences of essentially all of the flagellar genes are known. This proposal focuses on the process of flagellar assembly and on the attendant process of export of external components. The goal is to establish the components of the export apparatus (as opposed to the proteins exported), where the apparatus is located, and ultimately how it functions. Several candidates for export apparatus components exist, based on a miscellany of information deriving from previous studies. Basic information will be gathered as to the general cellular location and stoichiometry of putative components of the apparatus; evidence for interactions among them will be sought by intergenic suppression and affinity techniques. Several specific questions regarding process will be addressed, such as how the lengths of substructures like the flagellar hook are controlled, and whether there are scaffolding elements that are present during assembly and then discarded. How a cell assembles a complex organelle is a fundamental question, and the bacterial flagellum is an excellent system for its exploration. Thus the proposed research is of intrinsic biological interest and importance. Further significance derives from the recent realization that export of flagellar proteins and export of virulence factors by pathogenic bacteria like Yersinia pestis are somehow related. There are at least four putative export components of the two systems that are remarkably similar (up to 50% amino acid identity), a finding that is likely to have significant mechanistic and evolutionary implications. An understanding of how a pathogen exports molecules like invasins and adhesins could aid in designing clinical approaches for combating pathogens. Study of both systems is likely to be mutually beneficial in terms of understanding this special class of export process; the fact that the flagellar genetic system is better understood may enable it to provide a leading role in the investigation.